Corrosion of the metal strengthening elements used in the building of reinforced concrete structures causes progressive deterioration of the concrete to varying degrees. In its final stage, this deterioration appears in the form of cracks at the surface of the concrete or even splitting of the concrete mass. The difficulty lies in the fact that, in many cases, it is inadvisable to wait until this stage before acting, but, in the absence of any visible phenomenon, identification of the damaged zones of concrete becomes a matter of chance. At the present time, in fact, there are no non-destructive techniques which make it possible to locate the zones damaged by corrosion of the metal strengthening elements, even less the zones which are liable to be damaged in this way at points where corrosion of the said metal elements is still slight.
It is known that ferrous metals, for example steel, tend to corrode. This phenomenon involves an electrochemical process which depends particularly on the oxygen concentration, moisture content and acidity of the ambient medium, and which can be characterized by its electrical potential. In fact, it is known how to measure the corrosion potential of steel bars embedded in concrete by using a reference electrode such as a calomel electrode or an electrode of the Ag/AgCl or Cu/CuSO.sub.4 type.